Why some galaxies have this white color? As I see the most of them are irregular.
In the SDSS color scheme, 'white' means the light detected in each of the g, r, and i bands is approximately the same. For most parts of most galaxies - at least those without lots of dust - the galaxy light is basically the same as that of the stars and ionized gas of which the galaxy is made*. For 'white' galaxies, it's light dominated by stars of a particular kind ... for historical reasons these stars are called "A" stars (I'm simplifying a lot). A galaxy with lots of A stars - and not so many O or B ones, and relatively few G to M giants - is rather unusual.
It's a pity this particular galaxy has no spectrum; if it had, we could see whether it is, in fact, dominated by light from A stars.
You may find the GZ forum's Object of the Day thread entitled A Paler Shade of White interesting.
Happy (continued) hunting! 😃
*the light from galaxies with active nuclei - AGNs (quasars are one kind of AGN) - or at least from those nuclei also comes from something other than stars. Some AGNs can look white in SDSS images